High-cycle fatigue life prediction for Pb-free BGA under random vibration loading

Abstract

This paper develops an assessment methodology based on vibration tests and finite element analysis (FEA) to predict the fatigue life of electronic components under random vibration loading. A specially designed PCB with ball grid array (BGA) packages attached was mounted to the electro-dynamic shaker and was subjected to different vibration excitations at the supports. An event detector monitored the resistance of the daisy chained circuits and recorded the failure time of the electronic components. In addition accelerometers and dynamic signal analyzer were utilized to record the time-history data of both the shaker input and the PCB’s response. The finite element based fatigue life prediction approach consists of two steps: The first step aims at characterizing fatigue properties of the Pb-free solder joint (SAC305/SAC405) by generating the S– N (stress-life) curve. A sinusoidal vibration over a limited frequency band centered at the test vehicle’s 1st natural frequency was applied and the time to failure was recorded. The resulting stress was obtained from the FE model through harmonic analysis in ANSYS. Spectrum analysis specified for random vibration, as the second step, was performed numerically in ANSYS to obtain the response power spectral density (PSD) of the critical solder joint. The volume averaged Von Mises stress PSD was calculated from the FEA results and then was transformed into time-history data through inverse Fourier transform. The rainflow cycle counting was used to estimate cumulative damages of the critical solder joint. The calculated fatigue life based on the rainflow cycle counting results, the S– N curve, and the modified Miner’s rule agreed with actual testing results.